1. Field of the Invention
The present invention relates generally to strain measurement systems, and more particularly to a strain measurement module and strain measurement system using the strain measurement module, which is used to monitor a structure while collecting the strain information of the structure using a light generator and a fiberoptic sensor.
2. Description of the Related Art
A conventional strain gauge used to diagnose the condition of a structure is disadvantageous in that it does not have durability sufficient to be used for the diagnosis of the structure, a measurement cooper wire must be provided in each of sensors, and it may influence the structure in the case of many measurement points because power must be supplied to measure resistance. For these reasons, various attempts have been made to replace the conventional strain gauge system with the fiberoptic sensor.
FIG. 1 is a schematic diagram showing the construction of a conventional strain measurement system 100 using a fiberoptic sensor.
As shown in FIG. 1, the conventional strain measurement system 100 using the fiberoptic sensor includes a light generator 110, an optical detection unit 130, a compensation unit 140, a fiberoptic sensor unit 150 and a control unit 160.
The operation of the conventional strain measurement system 100 shown in FIG. 1 is described below.
A Light Emitting Diode (LED) driver 112 constituting a part of the light generator 110 supplies power to an LED 114 to generate light having a certain wavelength distribution. The generated light passes through a coupler 120 and proceeds to the fiberoptic sensor unit 150 attached to or embedded in a structure.
Although a variety of fiberoptic sensors may be used as the fiberoptic sensor unit 150, FIG. 1 depicts an example in which Fiber Bragg Grating (FBG) sensors are used. Each of the FBG sensors reflects wavelengths of a certain width satisfying the Bragg's condition and passes the remaining wavelengths therethrough.
The reflected light reflected by the FBG sensor because it satisfies a certain wavelength condition proceeds to the optical detection unit 130 through the coupler 120. The optical detection unit 130 passes only the reflected light of a certain wavelength therethrough using a Fabry-Perrot (FP) filter 134 and transfers the reflected light to an optical detector, such as a Photo Diode (PD) 136. The FP filter 134 is provided therein with a lead-zirconate titanate (PZT) element (not shown) to be synchronized with the wavelength of the reflected light. Through the adjustment of the length of the PZT element depending on the extension and contraction of a cavity located in the FP filter 134, the passage of the reflected light passes through the FP filter 134 is controlled. In order to control the extension and contraction of the PZT element as described above, the FP filter 134 is connected to a PZT driver 132.
As described above, the PD 136 measures and outputs the intensity of reflected light. While the output from the PD 136 passes through a differentiator and a comparator, the peak point of the reflected light is detected and intensity is calculated at the peak point. The calculated intensity is input to a Central Processing Unit (CPU) 166.
The CPU 166 detects the wavelength of the reflected light from a voltage value that is applied to the PZT driver 132 when the reflected light is detected. From the value of the detected wavelength, the variation of strain generated in the FBG sensor can be calculated.
To compensate for the non-linearity of a voltage-length relationship that the PZT element of the FP filter 134 has, the compensation unit 140 including an Ethalon filter 144 may be added to the system. The compensation unit 140 is constructed to include an LED 146, an LED driver 148, the Ethalon filter 144 and a compensation FBG 142, and is connected to the coupler 120. The light output from the LED 146 by the operation of the LED driver 148 is transferred to the coupler 120 through the Ethalon filter 144 and the compensation FBG 142. The optical detection unit 130 measures the intensity of light in the same manner as in the reception operation of the reflected light, and transfers the measured intensity to the CPU 166. The CPU 166 utilizes the output value detected in the optical detection unit 130 to compensate for the wavelengths of the reflected light transmitted from the fiberoptic sensor unit 150.
In the above-described system, the construction and operation of the FBG 142, the FP filter 134 and the Ethalon filter 144 are well known to those skilled in the art. Accordingly, detailed descriptions of those are omitted here.
Since the above-described conventional strain measurement system is provided with the LED having a low output that is used as a light signal, it is not easy to measure a signal. In particular, for architectural structures, the transmission distance of a signal is long, so that it is almost impossible to measure the signal. Furthermore, the conventional strain measurement system using the LED as a light source is disadvantageous in that it must be provided with a plurality of FP filters corresponding to a plurality of FBG sensors in the case where the plurality of FP filters are embedded at a plurality of locations.
In order to overcome the above-described problems, there was proposed another conventional strain measurement system equipped with a tunable laser generator in which a high output laser and an FP filter were disposed at a source stage. The tunable laser generator of this system uses an Erbium Doped Fiber Amplifier (EDFA) as an amplifying mechanism, which is illustrated in FIG. 2.
The operation of the tunable laser generator is described with reference to FIG. 2 below. Weak signal light of about 1550 nm and a laser beam of 1480 nm generated in a pump laser 210 are joined together in a multiplexer 220, and the joined signal light and laser beam are transferred to a fiberoptic amplifier 250. The laser beam transferred to the fiberoptic amplifier 250 excites erbium ions Er3+ to an upper level, while the signal light causes erbium ions to transition to a lower level. In this process, light of 1550 nm is induction-emitted and is joined with the signal light. The intensified signal light excites other erbium ions again so that further intensified light is emitted. The light amplified during circulation through the fiberoptic amplifier 250 passes through an FP filter 230 and is output as a laser signal having a certain wavelength.
As described above, the tunable laser generator overcomes a limitation in the transmission distance of a signal and simplifies the structure of a reception unit, but has many other problems.
The tunable laser generator is disadvantageous in that it must be provided with the laser diode and the multiplexer because it must use the laser beam as well as the signal light as input signals, an area that optical fiber occupies is large and, thus, causes the system to be complicated because amplification is performed in the optical fiber, and the temperature control of the laser generator is difficult. Furthermore, since the laser beam has high polarization and coherency compared with general light, an interference phenomenon is serious in an optical detector. Additionally, high manufacturing costs are incurred to apply the laser generator to the strain measurement system.
The strain measurement system using the tunable laser generator has a high output and can simplify the structure of the optical detector. However, the strain measurement system is problematic in that the structure of a source stage, that is, the laser generator, is complicated, the control of temperature is difficult, the fabrication of a high-precision system is difficult because a laser beam having high polarization and coherency is used as a signal, and the manufacturing costs of the system are high, compared with the strain measurement system using the LED as a light source. As a result, there has been a demand for a strain measurement system that is capable of overcoming the above-described problems.
Meanwhile, the two conventional strain measurement systems using optical fiber use the FP filter in the optical detector or laser generator. However, the FP filter is problematic in that it is sensitive to the variation of temperature. However, the conventional strain measurement systems do not provide any countermeasure to this problem.